On Fri, Oct 11, 2019 at 03:40:48PM +0200, Stefano Garzarella wrote: > On Sun, Sep 1, 2019 at 8:56 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > On Fri, Aug 30, 2019 at 11:40:59AM +0200, Stefano Garzarella wrote: > > > On Mon, Jul 29, 2019 at 10:04:29AM -0400, Michael S. Tsirkin wrote: > > > > On Wed, Jul 17, 2019 at 01:30:26PM +0200, Stefano Garzarella wrote: > > > > > Since virtio-vsock was introduced, the buffers filled by the host > > > > > and pushed to the guest using the vring, are directly queued in > > > > > a per-socket list. These buffers are preallocated by the guest > > > > > with a fixed size (4 KB). > > > > > > > > > > The maximum amount of memory used by each socket should be > > > > > controlled by the credit mechanism. > > > > > The default credit available per-socket is 256 KB, but if we use > > > > > only 1 byte per packet, the guest can queue up to 262144 of 4 KB > > > > > buffers, using up to 1 GB of memory per-socket. In addition, the > > > > > guest will continue to fill the vring with new 4 KB free buffers > > > > > to avoid starvation of other sockets. > > > > > > > > > > This patch mitigates this issue copying the payload of small > > > > > packets (< 128 bytes) into the buffer of last packet queued, in > > > > > order to avoid wasting memory. > > > > > > > > > > Reviewed-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx> > > > > > Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx> > > > > > > > > This is good enough for net-next, but for net I think we > > > > should figure out how to address the issue completely. > > > > Can we make the accounting precise? What happens to > > > > performance if we do? > > > > > > > > > > Since I'm back from holidays, I'm restarting this thread to figure out > > > how to address the issue completely. > > > > > > I did a better analysis of the credit mechanism that we implemented in > > > virtio-vsock to get a clearer view and I'd share it with you: > > > > > > This issue affect only the "host->guest" path. In this case, when the > > > host wants to send a packet to the guest, it uses a "free" buffer > > > allocated by the guest (4KB). > > > The "free" buffers available for the host are shared between all > > > sockets, instead, the credit mechanism is per-socket, I think to > > > avoid the starvation of others sockets. > > > The guests re-fill the "free" queue when the available buffers are > > > less than half. > > > > > > Each peer have these variables in the per-socket state: > > > /* local vars */ > > > buf_alloc /* max bytes usable by this socket > > > [exposed to the other peer] */ > > > fwd_cnt /* increased when RX packet is consumed by the > > > user space [exposed to the other peer] */ > > > tx_cnt /* increased when TX packet is sent to the other peer */ > > > > > > /* remote vars */ > > > peer_buf_alloc /* peer's buf_alloc */ > > > peer_fwd_cnt /* peer's fwd_cnt */ > > > > > > When a peer sends a packet, it increases the 'tx_cnt'; when the > > > receiver consumes the packet (copy it to the user-space buffer), it > > > increases the 'fwd_cnt'. > > > Note: increments are made considering the payload length and not the > > > buffer length. > > > > > > The value of 'buf_alloc' and 'fwd_cnt' are sent to the other peer in > > > all packet headers or with an explicit CREDIT_UPDATE packet. > > > > > > The local 'buf_alloc' value can be modified by the user space using > > > setsockopt() with optname=SO_VM_SOCKETS_BUFFER_SIZE. > > > > > > Before to send a packet, the peer checks the space available: > > > credit_available = peer_buf_alloc - (tx_cnt - peer_fwd_cnt) > > > and it will send up to credit_available bytes to the other peer. > > > > > > Possible solutions considering Michael's advice: > > > 1. Use the buffer length instead of the payload length when we increment > > > the counters: > > > - This approach will account precisely the memory used per socket. > > > - This requires changes in both guest and host. > > > - It is not compatible with old drivers, so a feature should be negotiated. > > > 2. Decrease the advertised 'buf_alloc' taking count of bytes queued in > > > the socket queue but not used. (e.g. 256 byte used on 4K available in > > > the buffer) > > > - pkt->hdr.buf_alloc = buf_alloc - bytes_not_used. > > > - This should be compatible also with old drivers. > > > > > > Maybe the second is less invasive, but will it be too tricky? > > > Any other advice or suggestions? > > > > > > Thanks in advance, > > > Stefano > > > > OK let me try to clarify. The idea is this: > > > > Let's say we queue a buffer of 4K, and we copy if len < 128 bytes. This > > means that in the worst case (128 byte packets), each byte of credit in > > the socket uses up 4K/128 = 16 bytes of kernel memory. In fact we need > > to also account for the virtio_vsock_pkt since I think it's kept around > > until userspace consumes it. > > > > Thus given X buf alloc allowed in the socket, we should publish X/16 > > credits to the other side. This will ensure the other side does not send > > more than X/16 bytes for a given socket and thus we won't need to > > allocate more than X bytes to hold the data. > > > > We can play with the copy break value to tweak this. > > > > Hi Michael, > sorry for the long silence, but I focused on multi-transport. > > Before to implement your idea, I tried to do some calculations and > looking better to our credit mechanism: > > buf_alloc = 256 KB (default, tunable through setsockopt) > sizeof(struct virtio_vsock_pkt) = 128 > > - guest (we use preallocated 4 KB buffers to receive packets, copying > small packet - < 128 -) > worst_case = 129 > buf_size = 4 KB > credit2mem = (buf_size + sizeof(struct virtio_vsock_pkt)) / worst_case = 32 > > credit_published = buf_alloc / credit2mem = ~8 KB > Space for just 2 full packet (4 KB) > > - host (we copy packets from the vring, allocating the space for the payload) > worst_case = 1 > buf_size = 1 > credit2mem = (buf_size + sizeof(struct virtio_vsock_pkt)) / worst_case = 129 > > credit_published = buf_alloc / credit2mem = ~2 KB > Less than a full packet (guest now can send up to 64 KB with a single > packet, so it will be limited to 2 KB) > > Current memory consumption in the worst case if the RX queue is full: > - guest > mem = (buf_alloc / worst_case) * > (buf_size + sizeof(struct virtio_vsock_pkt) = ~8MB > > - host > mem = (buf_alloc / worst_case) * > (buf_size + sizeof(struct virtio_vsock_pkt) = ~32MB > > I think that the performance with big packets will be affected, > but I still have to try. > > Another approach that I want to explore is to play with buf_alloc > published to the peer. > > One thing that's not clear to me yet is the meaning of > SO_VM_SOCKETS_BUFFER_SIZE: > - max amount of memory used in the RX queue > - max amount of payload bytes in the RX queue (without overhead of > struct virtio_vsock_pkt + preallocated buffer) > > From the 'include/uapi/linux/vm_sockets.h': > /* Option name for STREAM socket buffer size. Use as the option name in > * setsockopt(3) or getsockopt(3) to set or get an unsigned long long that > * specifies the size of the buffer underlying a vSockets STREAM socket. > * Value is clamped to the MIN and MAX. > */ > > #define SO_VM_SOCKETS_BUFFER_SIZE 0 > > Regardless, I think we need to limit memory consumption in some way. > I'll check the implementation of other transports, to understand better. SO_VM_SOCKETS_BUFFER_SIZE might have been useful for VMCI-specific applications, but we should use SO_RCVBUF and SO_SNDBUF for portable applications in the future. Those socket options also work with other address families. I guess these sockopts are bypassed by AF_VSOCK because it doesn't use the common skb queuing code in net/core/sock.c :(. But one day we might migrate to it... Stefan
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